Piezo-electric actuators are used in imaging systems to adjust the positions of zoom and focusing lens systems. Piezo-electric actuators are also used in other systems to move small objects. Driving devices providing suitable driving voltages are required to operate said piezo-electric actuators. Piezo-electric actuators are implemented by using one or more piezo-elements, which are deflected, expanded or contracted when coupled to an activating voltage.
Energy consumption related to the driving of piezo-elements is a problem especially in mobile systems. In mobile systems, the available power supply is typically a battery with a limited capacity. A piezo-element comprises a substantial internal capacitance and consequently it stores a certain amount of energy when it is coupled to a driving voltage. In order to improve the energy efficiency of a mobile device, it is advantageous to recover the capacitive energy stored in the piezo-elements.
Another aspect in mobile devices is that the voltage of an available power supply is typically lower than the optimum driving voltage of the piezo-elements. Driving voltages higher than the power supply voltage are typically generated using step-up means.
U.S. Pat. No. 6,563,251 discloses a driving device for an actuator having a capacitive motor phase, said driving device comprising a set of voltage sources, switching means connecting said voltage sources to said capacitive motor phase, one at a time, and a capacitive voltage step-up device, supporting or being itself said voltage sources.
The article “Efficient Charge Recovery Method for Driving Piezoelectric Actuators with Quasi-Square Waves”, D. Campolo & al., IEEE Transactions on ultrasonics, ferroelectrics, and frequency control, Vol. 50 No. 3, March 2003, discloses a driving circuit for two piezo-elements, said driving circuit comprising an inductive element arranged to transfer a charge from a first piezo-element to a second piezo-element. The energy stored in the piezo-elements is partly recovered and re-used by means of said inductive element.